1. Field of the Invention
The invention relates to a pressure indicator, more particularly to a tire pressure gauge which is provided with an alarm generating device so as to warn the driver upon detection of insufficient or excessive pressure in a tire.
2. Description of the Related Art
A pen-type conventional tire pressure indicator comprises a hollow elongated main casing having a first end, which is to be connected to the inflating valve of a tire so as to open the inflating valve, and a second end which has a central hole formed therethrough. A piston member is movably provided inside the main casing and is in tight contact with an interior wall surface of the main casing. A pressure indicating stick has a first end that is disposed inside the main casing adjacent to the piston member and a second end that extends out of the main casing via the central hole in the second end of the main casing. A biasing spring is provided between the piston member and the second end of the main casing.
When the main casing is attached to the inflating valve of a tire, air pressure from the tire pushes the piston member to move toward the second end of the main casing. The compression spring is compressed, and the pressure indicating stick extends through the second end of the elongated main casing by a length corresponding to the air pressure inside the tire. Since a scale is disposed along a longitudinal length of the pressure indicating stick, the user can easily read the amount of pressure.
A gauge-type conventional tire pressure indicator is generally similar to the pen-type tire pressure indicator except that the former has a circular casing which is disposed around the second end of the elongated main casing. The circular casing has a front side provided with a dial, a rear side opposite to the front side and a toothed wheel rotatably mounted therein. A scale indicator is fixed to the toothed wheel inside the circular casing. The pressure indicating stick employed in this type includes a rack which meshes with the toothed wheel. When the first end of the elongated main casing of the tire pressure indicator is attached to an inflated tire, the pressure indicating stick moves linearly in the elongated main casing so as to rotate the toothed wheel, thus moving the scale indicator in accordance with the incoming pressure.
The tire pressure is usually inspected whenever the driver feels that the tire pressure is insufficient or when a tire has been inflated and it is desired to determine if an excessive or insufficient tire pressure condition exists. The tire pressure is read at the pressure indicating stick or on the scale dial. In the event that the reading operation is carried out under poor visibility, such as where there is inadequate lighting, one would have the difficulty in determining the tire pressure.